Fuel cells are energy conversion systems that convert chemical energies of fuels directly into electric energies. Fuel cells have high energy efficiency, and are environmentally friendly in that they are substantially free from emission of pollutants. Therefore, fuel cells have become the focus of attention as alternative energy technology. Among such fuel cells, polymer electrolyte membrane fuel cells (PEMFCs) are particularly advantageous, because they have a low drive temperature, are free from leakage problems caused by the use of a solid electrolyte, and allow high-speed operation. Thus, PEMFCs are spotlighted as portable, automotive and household power sources.
As a polymer electrolyte membrane, Nafion, a perfluorinated polymer membrane developed by the Dupont Inc. has been widely used. The polymer membrane (Nafion) has a polytetrafluoroethylene (PTFE) backbone, and shows excellent quality, including mechanical properties and chemical stability, in the field of fuel cells. However, Nafion is problematic in that it is expensive due to its complicated manufacturing process, causes degradation in the quality of a direct methanol fuel cell (DMFC) due to a so-called methanol crossover phenomenon, and shows decreased conductivity at high temperature. Therefore, novel polymer membranes have been developed to substitute for Nafion.
The polymer electrolyte membrane used in a fuel cell should be stable under the conditions required for driving the fuel cell. Thus, the polymer that may be used in the electrolyte membrane is extremely limited to aromatic polyether (APE), or the like. When a fuel cell drives, the polymer membrane is decomposed due to hydrolysis, oxidation and reduction, resulting in degradation in the quality of the fuel cell. Therefore, polyarylene ether polymers, including polyetherketone and polyethersulfone, have been researched and developed for their application for a fuel cell, due to their excellent chemical stability and mechanical properties.
U.S. Pat. No. 4,625,000 discloses a process of post-sulfonation for polyethersulfone as a polymer electrolyte membrane. In the post-sulfonation process, a strong acid such as sulfuric acid is used as a sulfonating agent and sulfonic acid groups (—SO3H) are introduced randomly into the polymer backbone. Hence, it is difficult to control the distribution, position and number of the sulfonic acid groups.
Additionally, EP 1,113,517A2 discloses a polymer electrolyte membrane, which comprises a block copolymer having a sulfonic acid group-containing block and a sulfonic acid group-free block. The block copolymer comprising an aliphatic block and an aromatic block is subjected to post-sulfonation by using sulfuric acid. Therefore, there is a problem of decomposition of chemical bonds of the aliphatic polymer during the sulfonation. Moreover, because the sulfonic acid groups are randomly introduced into the ring that forms the aromatic block, it is difficult to control the position and number of sulfonic acid groups in the polymer backbone.
Meanwhile, an article written by Prof. Watanabe [Macromolecules 2003, 36, 9691-9693] and Japanese Laid-Open Patent No. 2003-147074 disclose a process for introducing sulfonic acid groups into the fluorene present in a fluorene compound-containing copolymer by using chlorosulfonic acid (ClSO3H) or sulfuric acid. In the above method, sulfonic acid groups are randomly introduced into the ring that forms the fluorene compound.
The aforementioned sulfonation processes for a polymer according to the prior art could not satisfy the physical properties of an electrolyte membrane, required for driving a fuel cell. More particularly, when the content of sulfonic acid groups (degree of sulfonation; DS) is increased, i.e. when the ion exchange capacity (IEC) of an electrolyte membrane is increased to 1.3 meq/g or more, in order to realize a proton conductivity similar to the proton conductivity of commercially available Nafion, water content and methanol content of the electrolyte membrane increase excessively, resulting in a significant drop in mechanical integrity of the electrolyte membrane (for example, dissolution of the electrolyte membrane into methanol).